Mercury constitutes the primary component for generating ultraviolet (UV) light in mercury vapor discharge lamps. A layer comprising a luminescent material, for example, a fluorescent powder, may be present on an inner wall of the discharge vessel for converting UV light to light having a different wavelength, for example, UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general illumination purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of a low-pressure mercury vapor discharge lamp is usually tubular and comprises both elongated and compact embodiments. Generally, the tubular discharge vessel of a compact fluorescent lamp has a collection of comparatively short straight parts of a comparatively small diameter, which straight parts are interconnected by means of bridge parts or via bent parts. Compact fluorescent lamps are usually provided with an (integrated) lamp base. In such embodiments of the low-pressure mercury vapor discharge lamp, the discharge vessel comprises electrodes for maintaining a discharge inside the discharge vessel during operation of the lamp. Alternatively, in electrodeless mercury vapor discharge lamps, electric energy is inductively or capacitively coupled into the discharge space.
The term “nominal operation” in the description of the present invention is used for indicating operating conditions in which the mercury vapor pressure in the discharge vessel is such that the lamp has a radiation output of at least 80% of the output during optimum operation, i.e. under operating conditions in which the mercury vapor pressure is optimal. Furthermore, the term “initial radiation output” in the description is defined as the radiation output of the discharge lamp one second after switching on the discharge lamp, and the “run-up time” is defined as the time required by the discharge lamp to achieve a radiation output of 80% of the output during optimum operation.
A low-pressure mercury vapor discharge lamp as described in the opening paragraph, hereinafter also referred to as vapor pressure-controlled lamp, is known from EP 0 136 866 B1. As compared with the discharge lamp containing only free mercury, an amalgam limits the mercury vapor pressure in the discharge vessel. This renders nominal operation of the lamp possible at comparatively high lamp temperatures such as may occur in the case of a high lamp load, or when the lamp is used in a closed or poorly ventilated luminaire. The amalgam comprises mercury and at least one low melting point metal selected from tin, lead, bismuth and indium.
In addition to the mercury vapor discharge lamp according to the prior art, lamps are known which do not only comprise a (main) amalgam, but also an auxiliary amalgam. Provided that the auxiliary amalgam contains sufficient mercury, the lamp will have a comparatively short run-up time. Upon switching on the lamp, the auxiliary amalgam is heated by the electrode so that it evolves a substantial portion of the mercury present therein comparatively quickly. It is desirable that the lamp should be out of operation for a sufficiently long time before it is switched on, so that the auxiliary amalgam is able to take up sufficient mercury. When the lamp has been out of operation for a relatively short period, the shortening effect on the run-up time is only weak. Furthermore, a drawback especially arises in long lamps for which relatively much time is required before the mercury evolved by the auxiliary amalgam has spread over the entire discharge vessel, so that such lamps show a bright zone near the auxiliary amalgam and a darker zone remote from the auxiliary amalgam during a period of a few minutes after switching on.
In addition, low-pressure mercury vapor discharge lamps are known which are not provided with an amalgam and contain exclusively free mercury. These lamps, hereinafter also referred to as mercury lamps, have the advantage that the mercury vapor pressure at room temperature and hence the initial radiation output are comparatively high. Moreover, the run-up time is relatively short. Furthermore, lamps of this type, which have a relatively long discharge vessel, have a substantially constant brightness throughout their length after switching on, because the mercury vapor pressure (at room temperature) is sufficiently high upon switching on. Nominal operation at comparatively high lamp temperatures can be achieved with a mercury lamp whose discharge space contains just enough mercury to establish a mercury vapor pressure at the operating temperature that is close to the optimum mercury vapor pressure. During the lifetime of the lamp, however, mercury is lost because this is bound, for example, on a wall of the discharge vessel and/or by emitter material. Consequently, in practice, such a lamp has only a limited lifetime. In mercury lamps, a quantity of mercury is therefore dosed which is considerably higher than the quantity required in the vapor phase during nominal operation. However, this has the drawback that the mercury vapor pressure is equal to the vapor saturation pressure associated with the temperature of the coldest spot in the discharge vessel. Since the vapor saturation pressure rises exponentially with the temperature, temperature variations that occur, for example, in a poorly ventilated luminaire or in the case of a high lamp load, lead to a decrease of the radiation output. At comparatively low ambient temperatures, the mercury vapor pressure decreases, which also leads to a decrease of the radiation output.
When reducing the input power of a vapor-controlled lamp for dimming the light output of the lamp, the operating temperature of the lamp decreases. Hence, the temperature of the amalgam decreases as well. During the time a mercury vapor discharge lamp with a Bi—In—Hg amalgam according to the prior art cools down, the amalgam enters a temperature region wherein the mercury vapor pressures drops significantly, which results in a corresponding decrease of the light output of the lamp. In addition, a shift in the color temperature of the light generated by the lamp may occur. These phenomena are especially detrimental when a mercury vapor discharge lamp is used for Liquid Crystal Display (LCD) backlighting, in which lamps may be dimmed in order to improve the picture quality, for example, during scanning operation of the lamps in order to reduce motion blur effects. A significant drop in the light output and a possible change of the color temperature of the light strongly reduce the picture quality.